If one of my channel subscribers un-subscribes from my channel, what will happen to their information in my reporting?

If a subscriber decides to un-subscribe from your channel, their engagement data will still exist but they will have "unsubscribe" as their name and "withheld" as their job title and company name. When you download the report, their information will be listed as "unsubscribe" in every column of the report except for the viewing data. You will still see the number of viewings (live and on-demand) as well as viewing duration for that person.